In case of a light pulse having a pulse time duration which is equal to or longer than a picosecond, an electric field amplitude waveform can be directly measured by using an oscilloscope. However, if the pulse time duration is shorter than a picosecond, the electric field amplitude waveform cannot be measured by using the oscilloscope.
In an autocorrelator, an ultrashort light pulse is split into two equal pulses. The two pulses are simultaneously input to a nonlinear crystal, and second harmonic wave light is generated. An intensity of the second harmonic wave light is measured as a function of a delay time between the two pulses. This leads to intensity correlation. However, in this intensity correlation method, the phase of the light pulse cannot be obtained.
As a method of measuring the spectral intensity and the spectral phase of an ultrashort light pulse, an electro-optical sampling (EOS) method has been known (refer to, for example, Non-Patent Document 1). In the EOS method, a measurement target pulse having an electric field E0(t) and a probe light pulse having a time-delay-controlled electric field Er(t−τ) are input to an electro-optical (EO) crystal, and a polarization rotation amount of the probe light pulse according to birefringence induced in the EO crystal by the measurement target pulse having the electric field E0(t) is measured. Namely, the EOS method is a method of measuring interference between the probe light pulse having Er(t−τ) and modulation light having Er(t−τ) E0(t) modulated by the measurement target pulse having E0(t).
In addition, recently, a frequency resolved optical gate (FROG) method for measuring the spectral intensity and phase of an ultrashort light pulse has been developed (refer to, for example, Patent Document 1). In the FROG method, a measurement target pulse having an electric field E0(t) and a reference light pulse having a time-delay-controlled electric field Er(t−τ) are input to a nonlinear medium to generate an induced diffraction grating in the nonlinear medium. The measurement target light E0(t) input to the nonlinear medium is diffracted by the reference light Er(t−τ), and the diffracted pulse light is spectrally split by a spectroscopic device, so that the intensity and the phase of the measurement target light are obtained.